In vehicle air conditioning systems, electric vehicles (EV) cannot use the waste heat of the engine for cabin heating. In hybrid vehicles (HEV, PHEV) also, engines are controlled to be stopped as much as possible for fuel economy, so the adoption of heat pump cabin heating systems using a refrigerant and heating systems using an electric heater with coolant as a medium is being investigated. In particular, for cabin heating operation, the power consumption is large, so it is desirable to use a heat pump system that can achieve high COP cabin heating (in the case of an electric heater, COP≤1).
On the other hand, in the case of heat pump systems, when operating cabin heating under low outside air temperature conditions, frosting can occur on the vehicle-cabin-exterior heat exchanger (evaporator), resulting in a reduction in the quantity of heat absorbed from the external air, which has the problem that the cabin heating performance is reduced. Therefore, it is necessary to defrost the vehicle-cabin-exterior heat exchanger (evaporator), but if a defrosting system adapted for building heat pump air conditioning apparatus is adopted, cabin heating operation cannot be continued. In the case of a vehicle air conditioning device, if the cabin heating operation is stopped, the windows become fogged and safety cannot be ensured, so there is a problem that it is difficult to carry out defrosting operation while driving.
Therefore, Patent Document 1 provided a heat-pump-type vehicle air conditioning system ideal for use in EV vehicles or in HEV or PHEV vehicles and the like, that uses an existing vehicle air conditioning device cabin cooling cycle, and that by adding the minimum cabin heating circuit and equipment thereto, can resolve the problem of frosting of the vehicle-cabin-exterior vaporizer at low cost. This system connects a vehicle-cabin-interior condenser provided on the downstream side of the vehicle-cabin-interior vaporizer within the HVAC unit of the cabin cooling cycle of an existing system, and connects a vehicle-cabin-exterior vaporizer outside the vehicle cabin to configure a heat pump cycle for cabin heating. When frosting is detected on the vehicle-cabin-exterior vaporizer during cabin heating operation, the refrigerant to the vehicle-cabin-exterior vaporizer is cut off and refrigerant is passed to the vehicle-cabin-interior vaporizer side, and by switching to dehumidifying heating, operation can be continued.